The present invention pertains to downhole sliding sleeve valves. An example of such a valve is that sold under the trademark "SLIDING SIDE DOOR" type XA by Otis Engineering, Corp. Such a valve typically has a tubular housing or main body which can be made up into a string of well conduit (typically production tubing, but conceivably drill pipe or some other conduit type) as part thereof. The valve may be used to selectively prevent or permit flow between the well annulus and the interior of the conduit string. For example, packers in the conduit string above and/or below the valve can be used to pack off or isolate a given zone of the well. The sleeve valve can be left closed to maintain that isolation, or when it is desired to produce from that zone, the sleeve valve can be opened to permit that. In other exemplary situations, the valve may be opened to permit a fluid to pass from the interior of the tubing string into the annulus.
The fluid in the isolated zone of the well may be under considerable pressure, and there may be a large pressure differential between the annulus in that zone and the interior of the conduit string. It is highly desirable that these pressures be equalized, as by allowing slow bleeding of pressurized fluid from the annulus into the conduit, before full production flow is established, so that there will not be a sudden surge of pressure into the conduit. Such a surge can be dangerous, for any number of reasons well known to those of skill in the art.
To this end, a typical such sliding sleeve valve has one or more flow ports extending radially through the housing wall. A valve element in the form of a sleeve carried co-axially within the housing for selective longitudinal movement has a closed position in which an unbroken (not perforated) portion of its seal section is aligned with the flow port(s) and sealed with respect to the housing at least by a primary seal spaced in a first longitudinal direction from the flow port(s), and typically also by a first auxiliary seal spaced from the flow port(s) in a second direction opposite the first. This sleeve has a relatively small pressure relief aperture through its seal section, but spaced from the aforementioned sealed off portion, more specifically, spaced in the first direction from the primary seal when the sleeve is in the closed position.
The sleeve can be moved longitudinally within the housing to an open position, more specifically a pressure relief position, by moving it in the second direction so that the pressure relief aperture crosses over to the opposite side of the primary seal and thereby becomes communicable with the flow port. This allows fluid to bleed from the annulus slowly through the pressure relief aperture until the pressure in the conduit is approximately equal to that in the annulus, without any sudden surge of pressure into the conduit. Thereafter, the sleeve can be further moved in the second direction to another open position, specifically a full flow position, in which one or more full flow openings in the sleeve, providing substantially greater flow area than the pressure relief aperture, are communicated with the flow port in the housing.
A common problem with such valves is that, when the sleeve is moving from the closed position to the pressure relief position, the large pressure differential between the annulus and the interior of the conduit is acting on the primary seal urging it radially inwardly tightly against the sleeve. Then, when the edges of the pressure relief aperture cross this seal which is being urged inwardly against them, they can literally clip off a substantial bit of the material of the seal, rendering that seal less effective, or even ineffective, for further sealing. This is particularly disadvantageous since, in many operations, it may be necessary to re-close and subsequently reopen the valve after it has been operated at least once before. Each such reopening may clip off another bit of seal material, so that even if the seal is not ruined on the first pass of the sleeve, it will eventually be ruined by subsequent passes.
The problem is further exacerbated where the primary seal, or at least its innermost portion, is elastomeric, e.g. an o-ring. Under pressure, the elastomeric material deforms and extrudes into the clearance between the sleeve and the housing, rendering an even greater volume of seal material vulnerable. The pressure relief aperture then passes the seal so quickly that, even after the leading edge of the aperture partially passes the seal, thus potentially allowing the elastomer to return to its relaxed configuration, there is not enough time for the elastomer to do that before the trailing edge of the aperture clips it.
There have been efforts to address somewhat similar problems in a downhole safety valve. Such a valve has an axially movable and axially seating valve element, such as a flapper or ball, which when closed, seals across the interior of the well conduit. An ancillary sleeve valve opens and closes a bypass around the main valve element. This valve had a similar problem with clipping of a seal ring when an aperture in the sliding sleeve passed thereacross. Efforts have been made to alleviate this by relieving or recessing the outer surface of the sliding sleeve in the vicinity of the aperture and/or by providing backup rings in the same retaining groove with the seal ring to attempt to prevent the seal ring from extruding into the gap between the sliding sleeve and the surrounding member. To the best knowledge of the present inventor, such devices not only continued to clip the seal ring, but in some instances, even clipped the backup rings.